1. Field of the Invention
The present invention relates to a spread illuminating apparatus of side light type, and more particularly to a side light type spread illuminating apparatus for use as a lighting means for a liquid crystal display device.
2. Description of the Related Art
Currently, a liquid crystal display (LCD) is extensively used as a display means for various electronic equipments. Since a liquid crystal of an LCD does not emit light by itself, a lighting means is required for achieving visibility when the LCD is used at night or in a dark place. A spread illuminating apparatus has conventionally been used as such a lighting means, and especially, a side light type spread illuminating apparatus is extensively used, which has a light guide plate having transparency, and a linear light source or one or more point light sources disposed at a side surface of the light guide plate. Recently, a spread illuminating apparatus incorporating a point light source, which enables simplification of a driving circuit, is seeing an increased usage in small-size electronic equipments such as mobile information terminals, and a white light emitting diode (hereinafter referred to simply as “LED”) is frequently employed as such a point light source.
In order to enhance the brightness for such a side light type spread illuminating apparatus, it is required to increase the luminance of an LED as a primary light source, and also to efficiently utilize light emitted from the LED. For increasing the luminance of the LED, current to drive the LED is raised to increase the amount of light emitted from an LED chip, but this causes the LED chip to generate increased heat so as to raise the ambient temperature consequently lowering the luminous efficiency of the LED chip and shortening the life of the LED chip, and facilitation of heat dissipation is required. On the other hand, the following problem is conventionally involved with regard to the efficient utilization of light emitted from the LED.
FIG. 4 is a top plan view of a conventional side light type spread illuminating apparatus. Referring to FIG. 4, a side light type spread illuminating apparatus 100 includes a light guide plate 101, and an LED 105 disposed at a side surface 102 (hereinafter referred to as “light entrance surface” as appropriate) of the light guide plate 101. In the spread illuminating apparatus 100, light emitted from the LED 105 is introduced in the light guide plate 101 through the light entrance surface 102 and is adapted to exit out uniformly from a major surface 104 while traveling from the light entrance surface 102 toward a side surface 103 opposite to the light entrance surface 102, thus providing function as a lighting means for a liquid crystal panel and the like. However, some light that is reflected at the side surface 103 and gets back to the light entrance surface 102 (such light is referred to as “internal reflection light”) leaks out from the light entrance surface 102 so as to take an optical path P1, thus causing loss light.
To deal with the problems described above, a spread illuminating apparatus using LEDs is disclosed (refer to, for example, Japanese Patent Application Laid-Open No. H9-298008; FIGS. 1 and 2 attached therein). FIG. 5 is a front elevation view of an LED structure 200 used in such a spread illuminating apparatus. The LED structure 200 includes a reflection case 201 formed of resin and having a rectangular solid configuration with its rear end blinded and its front end opened, and two LED chips 202 disposed at the blinded rear end of the reflection case 201. The LED structure 200 further includes terminal plates 203, 204 and 205 formed integrally with respective external lead terminals (not shown), and the two LED chips 202 are mounted to the terminal plate 204 by a bonding method, and are connected respectively to the terminal plates 203 and 205 by a wire bonding method. The aforementioned Japanese Patent Application Laid-Open No. H9-298008 describes a spread illuminating apparatus which includes two of the LED structures 200 disposed respectively at two shorter side surfaces of a substantially rectangular light guide plate, wherein the dimension of the reflection case 201 is optimally determined according to the dimension of the shorter side surface of the light guide plate.
Since the spread illuminating apparatus described above includes the terminal plates 203, 204 and 205 having a large area relative to the size of the LED chips 202, and includes the reflection case 201 having its longitudinal dimension measuring optimally up to the longitudinal dimension of the shorter side surface (as light entrance surfaces) of the light guide plate, some degree of favorable effects are anticipated on the above-described problems due to LED chips generating increased heat and due to internal reflection light leaking out from the light entrance surface.
In the LED structure 200 shown in FIG. 5, however, since the LED chips 202 are disposed at the blinded rear end of the reflection case 201 which is formed of resin so as to enclose the terminal plates 203, 204 and 205, the heat generated by the LED chips 202 and transmitted therefrom to the terminal plates 203, 204 and 205 cannot be sufficiently released outside due to the heat transfer resistance of the reflection case 201, though the terminal plates 203, 204 and 205 have a relatively large area for the size of the LED chips 202. Also, the light emitted from the LED chips 202 and the internal reflection light may possibly be absorbed by the reflection case 201 causing loss light, and thus efficient utilization of light emitted from the LED chips 202 cannot be achieved.